By way of introduction, differentiation is a process whereby structures and functions of cells are progressively committed to give rise to more specialised cells, such as the formation of T cells or B cells. Therefore, as the cells become more committed, they become more specialised.
In contrast, retro-differentiation is a process whereby structures and functions of cells are progressively changed to give rise to less specialised cells.
Undifferentiated cells are capable of multilineage differentiation—i.e. they are capable of differentiating into two or more types of specialised cells. A typical example of an undifferentiated cell is a stem cell.
In contrast, differentiated cells are incapable of multilineage differentiation. A typical example of a differentiated cell is a T cell.
There are many undifferentiated cells and differentiated cells found in vivo and the general art is replete with general teachings on them.
By way of example, reference may be made to inter alia Levitt and Mertelsman 1995 (Haematopoietic Stem Cells, published by Marcel Dekker Inc—especially pages 45–59) and Roitt et al (Immunology, 4th Edition, Eds. Roitt, Brostoff and Male 1996, Publ. Mosby—especially Chapter 10).
In short, however, examples of undifferentiated cells include lymphohaematopoietic progenitor cells (LPCs). LPCs include pluripotent stem cells (PSCs), lymphoid stem cells (LSCs) and myeloid stem cells (MSCs). LSCs and MSCs are each formed by the differentiation of PSCs. Hence, LSCs and MSCs are more committed than PSCs.
Examples of differentiated cells include T cells, B cells, eosinophils, basophils, neutrophils, megakaryocytes, monocytes, erythrocytes, granulocytes, mast cells, and lymphocytes.
T cells and B cells are formed by the differentiation of LSCs. Hence, T cells and B cells are more committed than LSCs.
Eosinophils, basophils, neutrophils, megakaryocytes, monocytes, erythrocytes, granulocytes, mast cells, NKs, and lymphocytes are formed by the differentiation of MSCs. Hence, each of these cells are more committed than MSCs.
Antigens are associated with undifferentiated and differentiated cells. The term “associated” here means the cells expressing or capable of expressing, or presenting or capable of being induced to present, or comprising, the respective antigen(s).
Most undifferentiated cells and differentiated cells comprise Major Histocompatability Complex (MHC) Class I antigens and/or Class II antigens. If these antigens are associated with those cells then they are called Class I+ and/or Class II+ cells.
Each specific antigen associated with an undifferentiated cell or a differentiated cell can act as a marker. Hence, different types of cells can be distinguished from each other on the basis of their associated particular antigen(s) or on the basis of a particular combination of associated antigens.
Examples of these marker antigens include the antigens CD34, CD19 and CD3. If these anigens are present then these particular cells are called CD34+, CD19+ and CD3+ cells respectively. If these antigens are not present then these cells are called CD34−, CD19− and CD3− cells respectively.
In more detail, PSCs are CD34+ cells. LSCs are DR+, CD34+ and TdT− cells. MSCs are CD34+, DR+, CD13+, CD33+, CD7+ and TdT+ cells. B CD19+, CD21+, CD22+ and DR+ cells. T cells are CD2−, CD3−, or CD8+ cells. Immature lymphocytes are CD4+ and CD8+ cells. Activated T cells are DR+ cells. Natural killer cells (NKs) are CD56+ and CD16+ cells. T lymphocytes are CD7+ cells. Leukocytes are CD45+ cells. Granulocytes are CD13+ and CD33− cells. Monocyte macrophage cells are CD14+ and DR+ cells.
Hence, by looking for the presence of the above-listed antigen markers it is possible to identify certain cell types (e.g. whether or not a cell is an undifferentiated cell or a differentiated cell) and the specialisation of that cell type (e.g. whether that cell is a T cell or a B cell).
The general concept of retrodifferentiation is not new. In fact, in 1976 Jose Uriel (Cancer Research 36, 4269–4275. November 1976) presented a review on this topic, in which he said:                “retrodifferentiation appears as a common adaptive process for the maintenance of cell integrity against deleterious agents of varied etiology (physical, chemical, and viral). While preserving the entire information encoded on its genome, cells undergoing retrodifferentiation lose morphological and functional complexity by virtue of a process of self-deletion of cytoplasmic structures and the transition to a more juvenile pattern of gene expression. This results in a progressive uniformization of originally distinct cell phenotypes and to a decrease of responsiveness to regulatory signals operational in adult cells. Retrodifferentiation is normally counterbalanced by a process of reontogeny that tends to restore the terminal phenotypes where the reversion started. This explains why retrodifferentiation remains invariably associated to cell regeneration and tissue repair.”        
Uriel (ibid) then went on to discuss cases of reported retrodifferentiation—such as the work of Gurdon relating to nuclei from gut epithelial cells of Xenopus tadpoles (Advances in Morphogenesis [1966] vol 4, pp 1–43. New York Academic Press, Eds Abercrombie and Bracher), and the work of Bresnick relating to regeneration of liver (Methods in Cancer Research [1971] vol 6, pp 347–391).
Uriel (ibid) also reported on work relating to isolated liver parenchymal cells for in vitro cultures. According to Uriel:                “Contrary to the results with fetal or neonatal hepatocytes, with hepatocytes from regenerating liver, or from established hepatomas, it has been difficult to obtain permanent class lines from resting adult hepatocytes.”        
Uriel (ibid) also reported on apparent retrodifferentiation in cancer, wherein he stated:                “the biochemical phenotypes of many tumours show analogous changes of reversion toward immaturity . . . during the preneoplastic phase of liver carcinogenesis, cells also retrodifferentiate.”        
More recent findings on retrodifferentiation include the work of Minoru Fukunda (Cancer Research [1981] vol 41, pp 4621–4628). Fukunda induced specific changes in the cell surface glycoprotein profile of K562 human leukaemic cells by use of the tumour-promoting phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA). According to Fukunda TPA appeared to induce the K562 human leukaemic cells into a retrodifferentiated stage.
Also, Hass et al (Cell Growth & Differentiation [1991] vol 2, pp 541–548) reported that long term culture of TPA-differentiated U-937 leukaemia cells in the absence of phorbol ester for 32–36 days resulted in a process of retrodifferentiation and that the retrodifferentiated cells detached from the substrate and reinitiated proliferation.
Another case of retrodifferentiation is the work of Curtin and Snell (Br. J. Cancer [1983] vol 48, pp 495–505]. These workers compared enzymatic changes occurring during diethylnitrosamine-induced hepatocarcinogenesis and liver regeneration after partial hepatectomy to normal liver differentiation. Theses workers found changes in enzyme activities during carcinogenesis that were similar to a step-wise reversal of differentiation. According to these workers, their results suggest that an underlying retrodifferentiation process is common to both the process of hepatocarcinogenesis and liver regeneration.
More recently, Chastre et al (FEBS Letters [1985] vol 188, number 2, pp 2810–2811] reported on the retrodifferentiation of the human colonic cancerous subclone HT29-18.
Even more recently, Kobayashi et al (Leukaemia Research [1994] vol 18, no. 12, pp 929–933) have reported on the establishment of a retrodifferentiated cell line (RD-1) from a single rat myelomonocyticleukemia cell which differentiated into a macrophage-like cell by treatment with lipopolysaccharide (LPS).
According to the current understanding, as borne out by the teachings found on page 911 of Molecular Biology of the Cell (pub. Garland Publishers Inc. 1983) and more recently Levitt and Mertelsman (ibid), a stem cell, such as a PSC, has the following four characteristics:                i. it is an undifferentiated cell—i.e. it is not terminally differentiated;        ii. it has the ability to divide without limit;        iii. it has the ability to give rise to differentiated progeny, such as the differentiated cells mentioned earlier; and        iv. when it divides each daughter has a choice: it can either remain as PSC like its parent or it can embark on a course leading irreversibly to terminal differentiation.        
Note should be made of the last qualification, namely that according to the general teachings in the art once an undifferentiated cell has differentiated to a more committed cell it can not then retrodifferentiate. This understanding was even supported by the teachings of Uriel (ibid), Fukunda (ibid), Hass et al (ibid), Curtin and Snell (ibid), Chastre et al (ibid), and Kobayashi et al (ibid) as these workers retrodifferentiated certain types of differentiated cells but wherein those cells remained committed to the same lineage and they did not retrodifferentiate into undifferentiated cells.